Forgotten multithreaded.frm in distribution

[tueda]

This PR simply adds multithreaded.frm to EXTRA_DIST (but it will lead to a conflict with #517).

Alternatively, it may not be necessary to add it to the distribution. In fact, make check does not run tests in subdirectories:

• checkpoint/checkpoint.frm
• forcer/forcer.frm
• multithreaded/multithreaded.frm

Unless we implement a comprehensive testing option like make check-full, these files may not be needed by users.

Consequently, we might consider removing these files from EXTRA_DIST instead.

[coveralls]

coverage: 48.776% (+0.02%) from 48.759%
when pulling 3f496b9 on tueda:fix-test-forgotten-multithreaded
into e58ebd3 on vermaseren:master.

[jodavies]

This would anyway need to be modified when I get around to doing #524 . Probably all available test files should come with the distribution, even if they are not run by default.

[jodavies]

Now this is resolved by #524.